Titanium wire and rod are extruded from billets produced by the Kroll process. The Kroll billets are batch produced and expensive. Wire and rod milling or extrusion from Kroll billets involve multi-step batch operations which result in low material yields and low product volumes. It would be desirable to be able to use cheaper feed material, in particular titanium powder, recycled chips, sponge granules or the like, and to be able to continuously form elongate titanium product, such as wire and rod, in increased product volume, in less processing steps with increased yields and lower process costs.                Tantalum and niobium are processed via primary and secondary routes. Primary processing is at very high temperatures and requires annealing, melting (or sintering) under high vacuum to prevent reaction with oxygen and nitrogen that diffuse into the matrix and result in embrittlement. The primary route involves the fabrication of billet by vacuum melting methods above 30000 (2415° C. for niobium) or from cold or hot pressed powder that is sintered at 20000 under vacuum.        Secondary processing is typically at room temperature with intermediate recrystallisation anneals after about 90% reduction to produce small bars for wire drawing. Conventional processing is thus segmented by batch processes. It would be desirable, therefore, to produce the final tantalum or niobium wire or rod in a single operation in a continuous manner and at lower temperatures.        
A known process for the production of aluminium and copper extrusions utilises a continuous rotary extrusion operation. The main process utilising rotary extrusion is the “Conform” process. The Conform process has a number of variants such as those based on Holton and BWE machines but these hold to the same principles of continuous rotary extrusion. A recent variant differs slightly from conventional continuous rotary extrusion in that its primary purpose is to provide severe plastic deformation of feed material without significant alteration of the feed cross section.
The rotary extrusion processes use apparatus of a type consisting of an extrusion system having a rotatable wheel and a fixed shoe which covers part of the length of a groove around the periphery of the wheel, to form an arcuate passageway. The shoe has an abutment which closes one end of the passageway, while a shaping or extrusion die spaced from the abutment by a die chamber defines a die orifice. The feed material at ambient temperature is presented to an inlet to the passageway, at the end of the passageway remote from the abutment, and rotation of the wheel causes the feed material to be drawn along the passageway by frictional engagement with the wheel exceeding frictional engagement with the shoe. The feed material is sufficiently subjected to heat and pressure by the frictional engagements to enable the feed material to be forced into the die chamber by engagement with the abutment and extruded through the die orifice. Examples of variants of apparatus of that type and processes using them are provided in the following US patents:                a) U.S. Pat. Nos. 3,765,216 and 3,872,703 to Green, 4,101,253 to Etherington, 4,044,587 to Green et al, 4,055,979 to Hunter et al and 4,061,011 to Green deceased et al, all assigned to United Kingdom Atomic Energy Authority;        b) U.S. Pat. No. 4,041,745 to Moreau, assignor to Trefimetaux;        c) U.S. Pat. No. 4,552,520 to East et al, assignors to Metal Box Public Ltd Co;        d) U.S. Pat. No. 4,650,408 to Anderson et al, assignors to Babcock Wire Equipment Ltd;        e) U.S. Pat. Nos. 5,167,138, 5,284,428 and 5,503,796, all to Sinha et al, assignors to The Southwire Co; and        f) U.S. Pat. No. 7,152,448 to Zhu et al, assignors to Los Alamos National Security, LLD.        
Each of these patents disclose continuous extrusion of feed material, of elongate solid feedstock or powder. The metal, when identified, typically is copper and or aluminium or their alloys, with extrusion commencing with feed material at ambient temperature.
While rotary extrusion processes are used for producing lengths of copper and aluminium products, this is not without limits on cross-sectional sizes and other difficulties. However, conditions suitable for operation with copper or aluminium feed materials have not been found to be suitable for use with high temperature formable non-ferrous metals susceptible to embrittlement during elevated temperature processing in air, such as titanium, tantalum and niobium feed materials.
USP '253 to Etherington points to the development of fatigue failure of the wheel due to fluctuating stresses which reach a maximum immediately in front of the abutment member, but falling to a minimum beyond the abutment member. Also, USP '745 to Moreau refers to the large and sometimes prohibitive amount of heat generated by friction between the feed material and walls of the passageway. Also, dead zones can form in the vicinity of the die, while energy yield is small due to absorption of motive energy by heating due to friction. Moreau states that for some metals various drawbacks are serious enough to make the process quite unsuitable. We have found this to be the case when conventional Conform processing is attempted with high temperature formable non-ferrous metals such as titanium, tantalum and niobium that are susceptible to embrittlement in elevated temperature processing in air.
US '979 to Hunter et al affirms the finding of Etherington in relation to straining that the passageway suffers when its walls are subjected to cyclic stressing as the feedstock is compressed. The mechanism is suggested by Hunter et al to be that the pressure/temperature cycle experienced by the wheel as it rotates, resulting in micro fatigue cracks in all the groove surfaces. We believe this problem is exacerbated with titanium, tantalum and niobium, and other metals to which the present invention extends, due to the substantially higher pressure levels necessary for its extrusion. USP '520 to East et al outlines the further problem of metal or “flash” extruded through necessary working clearances near the outlet end of the passageway. In addition to necessitating stoppages for removal of the flash, it increases frictional drag on the wheel and adds to the heat generated by friction and the operating temperature of various parts of the wheel and shoe members. USP '138, '428 and '796 each to Sinha et al attest to considerable heat generated by the enormous frictional resistance and resulting axial stress encountered by the feed material. Also, USP '138 refers to this real disadvantage in the context of stating that the advantages of the conform extrusion machine include the provision of a theoretically continuous extruding process and the use of cold solid or powdered feed material with avoidance of any need to preheat the material prior to its extrusion. That is, the problem of considerable heat generated by enormous frictional resistance despite the feed material mutually being at ambient temperature. Also, the conform apparatus is found according to USP '138 to produce extruded products having non-uniform microstructure and large surface grains which can cause “orange peel” when subjected to high stress working operations. USP '428 and '796 add to this problem recognition that, with extrusion of powder material, the conform apparatus can give rise to the serious problem of uneven powder flow due to flow turbulence and shear forces across the passageway. This is due to the shearing forces being higher along the extrusion shoe, which is fixed relative to the feed material, than along the grooved rotating wheel. Thus, differential cooling along the passageway can be necessary, and this would further complicate extrusion from titanium, tantalum and niobium powders.
USP '448 to Zhu et al extends the range of apparatus providing severe plastic deformation (SPD) by the technique of equal channel angular pressing (ECAP) or extrusion (ECAE). Specifically, Zhu et al proposes apparatus providing operation of the ECAP/ECAE technique with the continuous rotary extrusion enabled by Conform apparatus. The combined apparatus is illustrated as applicable to processing of aluminium bar. While the apparatus is said to be able to be used with any metal or alloy work piece, the degree of efficacy of the apparatus with other metals, such as titanium, and others to which the present invention extends, is not apparent.
Also of relevance is the article “Precise Extrusion Technology by Conform Process for Irregular Sectional Copper” by Tonogi et al, Hitachi Cable Review No-21, August 2002, pages 77 to 82, available at http://www.hitachi-cable.co.jp/en/about/publish/review/_icsFiles/afieldfile/2005/11/29/2_review13.pdf. This work points out that, for metals harder than aluminium, problems such as insufficient tool strength and inferior product quality, have limited Conform extrusions range of application. However, improvements to machinery have made it possible to use Conform extrusion for mass production of copper objects.